Using Message Contracts

Typically when building Windows Communication Foundation (WCF) applications, developers pay close attention to the data structures and serialization issues and do not need to concern themselves with the messages in which the data is carried. For these applications, creating data contracts for the parameters or return values is straightforward. (For more information, see Specifying Data Transfer in Service Contracts.)

However, sometimes complete control over the structure of a SOAP message is just as important as control over its contents. This is especially true when interoperability is important or to specifically control security issues at the level of the message or message part. In these cases, you can create a message contract that enables you to use a type for a parameter or return value that serializes directly into the precise SOAP message that you need.

This topic discusses how to use the various message contract attributes to create a specific message contract for your operation.

Using Message Contracts in Operations

WCF supports operations modeled on either the remote procedure call (RPC) style or the messaging style. In an RPC-style operation, you can use any serializable type, and you have available to you the features that are available to local calls, such as multiple parameters and ref and out parameters. In this style, the form of serialization chosen controls the structure of the data in the underlying messages, but the WCF runtime creates the messages themselves to support the operation. This enables developers who are not familiar with SOAP and SOAP messages to quickly and easily create and use service applications.

The following code example shows a service operation modeled on the RPC style.

Normally, a data contract is sufficient to define the schema for the messages. For instance, in the preceding example, it is sufficient for most applications if BankingTransaction and BankingTransactionResponse have data contracts to define the contents of the underlying SOAP messages. For more information about data contracts, see Using Data Contracts.

However, occasionally it is necessary to precisely control how a type is mapped to a SOAP message transmitted over the wire. The most common scenario for this is inserting custom SOAP headers. Another common scenario is to define security properties for the message's headers and body, that is, to decide whether these elements are digitally signed and encrypted. Messaging-style operations provide this control.

A messaging-style operation has at most one parameter and one return value where both types are message types; that is, they serialize directly into a specified SOAP message structure. This may be any type marked with the MessageContractAttribute or the Message type. The following code example shows an operation similar to the preceding RCP-style, but which uses the messaging style.

For example, if BankingTransaction and BankingTransactionResponse are both types that have message contracts, then the code in the following operations is valid.

[OperationContract]
bool Validate(BankingTransaction bt);
//Invalid - the return type is not a message contract.
[OperationContract]
void Reconcile(BankingTransaction bt1, BankingTransaction bt2);
//Invalid – there is more than one parameter.

An exception is thrown for any operation that involves a message contract type and that does not follow one of the valid patterns. Of course, operations that do not involve message contract types are not subject to these restrictions.

If a type has both a message contract and a data contract, only its message contract is considered when the type is used in an operation.

Defining Message Contracts

To define a message contract for a type (that is, to define the mapping between the type and a SOAP envelope), apply the MessageContractAttribute to the type. Then apply the MessageHeaderAttribute to those members of the type you want to make into SOAP headers, and apply the MessageBodyMemberAttribute to those members you want to make into parts of the SOAP body of the message.

When using this type as an operation parameter, a SOAP envelope is generated with extra headers called operation and transactionDate, which contain the contents of the operation and transactionDate fields. The SOAP body consists of a wrapper element containing elements called sourceAccount and targetAccount, of the Account data contract type and the amount of the integer type.

You can apply the MessageHeaderAttribute and MessageBodyMemberAttribute to all fields, properties, and events, regardless of whether they are public, private, protected, or internal.

Note:

KnownTypeAttribute attributes are ignored in message contracts. If a KnownTypeAttribute is required, place it on the operation that is using the message contract in question.

Using Custom Types Inside Message Contracts

Each individual message header and message body part is serialized (turned into XML) using the chosen serialization engine for the service contract where the message is used. The default serialization engine, the XmlFormatter, can handle any type that has a data contract, either explicitly (by having the System.Runtime.Serialization.DataContractAttribute) or implicitly (by being a primitive type, having the System.SerializableAttribute, and so on). For more information, see Using Data Contracts.

In the preceding example, the Operation and BankingTransactionData types must have a data contract, and transactionDate is serializable because DateTime is a primitive (and so has an implicit data contract).

However, it is possible to switch to a different serialization engine, the XmlSerializer. If you make such a switch, you should ensure that all of the types used for message headers and body parts are serializable using the XmlSerializer.

Using Arrays Inside Message Contracts

You can use arrays of repeating elements in message contracts in two ways.

The first is to use a MessageHeaderAttribute or a MessageBodyMemberAttribute directly on the array. In this case, the entire array is serialized as one element (that is, one header or one body part) with multiple child elements. Consider the class in the following example.

The default name for array entries is the name of the member to which the MessageHeaderArrayAttribute attributes is applied.

The MessageHeaderArrayAttribute attribute inherits from the MessageHeaderAttribute. It has the same set of features as the non-array attributes, for example, it is possible to set the order, name, and namespace for an array of headers in the same way you set it for a single header. When you use the Order property on an array, it applies to the entire array.

You can apply the MessageHeaderArrayAttribute only to arrays, not collections.

Using Byte Arrays in Message Contracts

Byte arrays, when used with the non-array attributes (MessageBodyMemberAttribute and MessageHeaderAttribute), are not treated as arrays but as a special primitive type represented as Base64-encoded data in the resulting XML.

When you use byte arrays with the array attribute MessageHeaderArrayAttribute, the results depend on the serializer in use. With the default serializer, the array is represented as an individual entry for each byte. However, when the XmlSerializer is selected, (using the XmlSerializerFormatAttribute on the service contract), byte arrays are treated as Base64 data regardless of whether the array or non-array attributes are used.

Signing and Encrypting Parts of the Message

A message contract can indicate whether the headers and/or body of the message should be digitally signed and encrypted.

For these security features to work, you must properly configure the binding and behaviors. If you use these security features without the proper configuration (for example, attempting to sign a message without supplying your credentials), an exception is thrown at validation time.

For message headers, the protection level is determined individually for each header.

For message body parts, the protection level can be thought of as the "minimum protection level." The body has only one protection level, regardless of the number of body parts. The protection level of the body is determined by the highest ProtectionLevelproperty setting of all the body parts. However, you should set the protection level of each body part to the actual minimum protection level required.

In this example, the recordID header is not protected, patientName is signed, and SSN is encrypted and signed. At least one body part, medicalHistory, has EncryptAndSign applied, and thus the entire message body is encrypted and signed, even though the comments and diagnosis body parts specify lower protection levels.

Controlling Header and Body Part Names and Namespaces

In the SOAP representation of a message contract, each header and body part maps to an XML element that has a name and a namespace.

By default, the namespace is the same as the namespace of the service contract that the message is participating in, and the name is determined by the member name to which the MessageHeaderAttribute or the MessageBodyMemberAttribute attributes are applied.

In this example, the IsAudited header is in the namespace specified in the code, and the body part that represents the theData member is represented by an XML element with the name transactionData.

Controlling Whether the SOAP Body Parts Are Wrapped

By default, the SOAP body parts are serialized inside a wrapped element. For example, the following code shows the HelloGreetingMessage wrapper element generated from the name of the MessageContractAttribute type in the message contract for the HelloGreetingMessage message.

To suppress the wrapper element, set the IsWrapped property to false. To control the name and the namespace of the wrapper element, use the WrapperName and WrapperNamespace properties.

Note:

Having more than one message body part in messages that are not wrapped is not compliant with WS-I Basic Profile 1.1 and is not recommended when designing new message contracts. However, it may be necessary to have more than one unwrapped message body part in certain specific interoperability scenarios. If you are going to transmit more than one piece of data in a message body, it is recommended to use the default (wrapped) mode. Having more than one message header in unwrapped messages is completely acceptable.

SOAP Header Attributes

The SOAP standard defines the following attributes that may exist on a header:

Actor/Role (Actor in SOAP 1.1, Role in SOAP 1.2)

MustUnderstand

Relay

The Actor or Role attribute specifies the Uniform Resource Identifier (URI) of the node for which a given header is intended. The MustUnderstand attribute specifies whether the node processing the header must understand it. The Relay attribute specifies whether the header is to be relayed to downstream nodes. WCF does not perform any processing of these attributes on incoming messages, except for the MustUnderstand attribute, as specified in the "Message Contract Versioning" section later in this topic. However, it allows you to read and write these attributes as necessary, as in the following description.

The second way to control these attributes is dynamically, through code. You can achieve this by wrapping the desired header type in the MessageHeader type (be sure not to confuse this type with the nongeneric version) and by using the type together with the MessageHeaderAttribute. Then, you can use properties on the MessageHeader to set the SOAP attributes, as shown in the following code example.

[C#]
[MessageHeader(MustUnderstand=true)] public MessageHeader<Person> documentApprover;
//… later on in the code:
BankingTransaction bt = new BankingTransaction();
bt.documentApprover= new MessageHeader<Person>();
bt.documentApprover.MustUnderstand=false; //override the static default of 'true'

Creating repeated headers with dynamic attribute control is allowed, as shown in the following code.

On the receiving side, reading these SOAP attributes can only be done if the MessageHeader class is used for the header in the type. Examine the Actor, Relay, or MustUnderstand properties of a header of the MessageHeader type to discover the attribute settings on the received message.

When a message is received and then sent back, the SOAP attribute settings only go round-trip for headers of the MessageHeader type.

Message Contract Versioning

Occasionally, you may need to change message contracts. For example, a new version of your application may add an extra header to a message. Then, when sending from the new version to the old, the system must deal with an extra header, as well as a missing header when going in the other direction.

The following rules apply for versioning headers:

WCF does not object to the missing headers—the corresponding members are left at their default values.

WCF also ignores unexpected extra headers. The one exception to this rule is if the extra header has a MustUnderstand attribute set to true in the incoming SOAP message—in this case, an exception is thrown because a header that must be understood cannot be processed.

Inheritance Considerations

A message contract type can inherit from another type, as long as the base type also has a message contract.

When creating or accessing a message using a message contract type that inherits from other message contract types, the following rules apply:

All of the message headers in the inheritance hierarchy are collected together to form the full set of headers for the message.

All of the message body parts in the inheritance hierarchy are collected together to form the full message body. The body parts are ordered according to the usual ordering rules (by System.ServiceModel.MessageBodyMemberAttribute.Orderproperty and then alphabetical), with no relevance to their place in the inheritance hierarchy. Using message contract inheritance where message body parts occur at multiple levels of the inheritance tree is strongly discouraged. If a base class and a derived class define a header or a body part with the same name, the member from the base-most class is used to store the value of that header or body part.

The PatientRecord class describes a message with one header called ID. The header corresponds to the personID and not the patientID member, because the base-most member is chosen. Thus, the patientID field is useless in this case. The body of the message contains the diagnosis element followed by the patientName element, because that is the alphabetical order. Notice that the example shows a pattern that is strongly discouraged: both the base and the derived message contracts have message body parts.

WSDL Considerations

When generating a Web Services Description Language (WSDL) contract from a service that uses message contracts, it is important to remember that not all message contract features are reflected in the resulting WSDL. Consider the following points:

WSDL cannot express the concept of an array of headers. When creating messages with an array of headers using the MessageHeaderArrayAttribute, the resulting WSDL reflects only one header instead of the array.

The resulting WSDL document may not reflect some protection-level information.

The message type generated in the WSDL has the same name as the class name of the message contract type.

When using the same message contract in multiple operations, multiple message types are generated in the WSDL document. The names are made unique by adding the numbers "2", "3", and so on, for subsequent uses. When importing back the WSDL, multiple message contract types are created and are identical except for their names.

SOAP Encoding Considerations

WCF allows you to use the legacy SOAP encoding style of XML, however, its use is not recommended. When using this style (by setting the Use property to Encoded on the System.ServiceModel.XmlSerializerFormatAttribute applied to the service contract), the following additional considerations apply:

The message headers are not supported; this means that the attribute MessageHeaderAttribute and the array attribute MessageHeaderArrayAttribute are incompatible with SOAP encoding.

If the message contract is not wrapped, that is, if the property IsWrapped is set to false, the message contract can have only one body part.

The name of the wrapper element for the request message contract must match the operation name. Use the WrapperName property of the message contract for this.

The name of the wrapper element for the response message contract must be the same as the name of the operation suffixed by 'Response'. Use the WrapperName property of the message contract for this.

After serializing the message using SOAP encoding, changedFrom and changedTo do not contain their own copies of p, but instead point to the copy inside the changedBy element.

Performance Considerations

Every message header and message body part is serialized independently of the others. Therefore, the same namespaces can be declared again for each header and body part. To improve performance, especially in terms of the size of the message on the wire, consolidate multiple headers and body parts into a single header or body part. For example, instead of the following code:

Event-based Asynchronous and Message Contracts

The design guidelines for the event-based asynchronous model state that if more than one value is returned, one value is returned as the Result property and the others are returned as properties on the EventArgs object. One result of this is that if a client imports metadata using the event-based asynchronous command options and the operation returns more than one value, the default EventArgs object returns one value as the Result property and the remainder are properties of the EventArgs object.

If you want to receive the message object as the Result property and have the returned values as properties on that object, use the /messageContract command option. This generates a signature that returns the response message as the Result property on the EventArgs object. All internal return values are then properties of the response message object.